A wide variety of ligand-receptor assays have been used to detect the presence of targets of interest in bodily fluids, such as serum, urine or saliva. These assays generally involve the capture and tagging of the target of interest on a solid substrate through antibody-antigen interactions, followed by signal generation with enzymatic, colorimetric, fluorescent or radioactive conjugates. These tests are relatively inexpensive, reliable, and easy to conduct. However, the sensitivity of lateral flow assays is frequently compromised by the formation of aggregates between conjugates and samples that prevent the labeled analytes from reaching the test zone for detection. Furthermore, the sensitivity of lateral flow assays is reduced when there is excess liquid, which floods the test strip and leads to insufficient reaction. The two issues mentioned above are particularly evident when dealing with samples containing salivary fluid.
However, unlike the other fluid specimens, salivary fluid cannot be applied directly to commercially available blood or urine lateral flow test strips because saliva samples do not flow easily. Further, saliva samples cause the detector colloidal particles to non-specifically adhere to the nitrocellulose membrane, thereby causing false results. This peculiar behavior is believed to be due to the presence of high concentrations of mucin and other highly viscous substances in the salivary fluid. Despite of its easy accessibility, salivary fluid usually contains very small amount of immunoglobulins. Compared to serum, the IgG and IgM levels in saliva are lower by a few orders of magnitude. Therefore, in order to obtain accurate results, a large volume of saliva sample is required for each test. However, conventional lateral flow devices are often not designed to handle large sample volumes. If too much sample is applied, it will result in overflow of the fluid and cause flooding of the test strip, bringing about disadvantages as discussed above.
Further, in most existing saliva-based tests, samples are collected and processed separately before being introduced to the assay. The salivary fluid is either collected in liquid form by drooling into containers, sometimes with the assistance of capillary tubes and pipettes, or collected on a solid substrate by chewing on paraffin, foam, cotton swab or other liquid-absorbent materials. Samples collected in liquid form are then centrifuged to remove the cellular components in order to obtain the saliva supernatant. Samples collected on solid substrates are usually eluted in buffers that are compatible with the subsequent assay desired. Samples collected on solid substrates may also be released into containers by pressurizing the solid materials. Such complicated sample collection and pretreatment procedures reduce the applicability of the salivary fluidics in lateral flow tests. Further, in one case, the saliva samples require a separate preparation step before they can be introduced to the lateral flow device. In another case, saliva samples can be collected with an integrated device, but no sample treatment step is demonstrated, hence it is not suitable for lateral flow-based saliva analysis. In yet another case, the saliva sample can be directly introduced to the lateral flow device, and an in-line sample pretreatment method is available to bridge the raw saliva sample to the lateral flow test strip, but the device is not able to process large sample volume, and the use of mucolytic agents may interfere with the lateral flow assay. In still another case, salivary fluid is collected onto a sorbent pad by swabbing the gum. The sample-carrying pad, which is directly coupled to the test strip, is subsequently submerged in the desired buffer to allow reactions to occur. However, the test strip does not assist in the flow of the sample as the sample has to flow against gravity. In yet another case, samples and reagents have to be introduced into a lateral flow assay separately, after one has completely migrated to the test zone, and the contact point of the sample and reagent at the test zone cannot be controlled or altered, resulting in inflexibility. Some designs of lateral flow devices may also not be streamlined, thereby resulting in difficulty of handling.
Accordingly, there is a need to provide alternative devices to analyze samples.